B.R.P. Rotax engines

Discussion in 'Propulsion' started by fpjeepy05, Mar 29, 2011.

  1. fpjeepy05
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    fpjeepy05 Senior Member

    These are the engines used in the sea-doo PWC's and Jet boats. I was wondering if anyone has ever used these with a conventional prop setup.

    I saw this. A 100hp Weber w/ Jet in a 16ft tender.
    http://www.cboatworks.com/specifications_16.html

    And this... http://www.youtube.com/watch?v=hr2GSTvXPt0

    Got me thinking. the Rotax 4tec puts out 255hp in a rather lightweight package, seems like a great motor for small planning craft that aren't attached to the Outboard stampede, or the heavy slugish overized 3.0L Mercs, or 350 Chevys.

    How bout the Weber? Anyone got anything on that? 8000 rpm max scares me. And I read the manual and they suggest fresh water flushing after ever use. Try selling that boat/motor to a customer and telling them that it is better than an outboard... but you have to wash it ever time you use it. haha. I'm sure they would love that. What is the warranty period in terms of hours on these type motors (pwc motors)? I know some outboards are around 2,000 hrs, but at 8,000 rpm... I would think warranty covers the first 10hrs.
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    As aeroplane engines (in ultralights) they do 100hrs (full service time) if you are lucky.
    For conventional prop you would need a custom reduction gear. As they are used in waterjets the high rews work better.
    Maybe usefull for surface piercing/super cavitating setups? Dunno much about them..
     
  3. dinoa
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    dinoa Senior Member

    I'd go with the Weber. Lots of them used on snowmobiles too. Look up their site. They are designed as a multipurpose engine. There are several units that have been modified and used to power light aircraft with good results.

    Rotax also make a fine engine. Most aircraft use 2 strokes that have an overhaul period of 300hrs. The highest power aircraft 2 stroke produced was 72 hp and is now out of production. Their four strokes are 80-115 hp with four horizontally opposed cylinders. They are considered very reliable with TBO of around 2000hrs. The four strokes are expensive and not suitable for marine applications because they have an integrated gear box and both air and water cooling.

    Also look at Yamahas. They make a 120hp 4 stroke also used in snowmobiles that is modified for aircraft.

    Dino
     
  4. fpjeepy05
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    fpjeepy05 Senior Member


    Does Yamaha make a marine version? Whats unappealing about the Rotax compared with the Weber? Whats the service period on the Weber?

    I was thinking of running a belt between the transmission and the motor. 2:1 reduction with the belt and another 2:1 reduction with the gear. Brings Shaft RPM down to around 2,000 rpm
     
  5. dinoa
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    dinoa Senior Member

    The Rotax TBOs are from their aircraft motors that have published overhaul periods. The others, including Rotax marine applications, don,t.

    I don't know about the marine Yamahas but the ones they use in aircraft are 4 stroke, 3 cylinder, 120hp Vector snowmobile engines. It would seem logical that Yamaha has something similar offered in a marine application.
    Dino
     
  6. fpjeepy05
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    fpjeepy05 Senior Member

    http://www.yamaha-motor.com/boat/advantage/Boat_advantage.aspx

    You're right. Not much information about them tho. Any of these motors avaiable as bobtails?
     
  7. fpjeepy05
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    fpjeepy05 Senior Member

    Why don't pwc engine manufacturers sell there engines? BRP's rotax 1503 scic puts out 255hp in a 300 pound package, compared to Mercrusiers 3.0L which puts out 135hp at 635lbs. I don't get it.
     
  8. powerabout
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    powerabout Senior Member

    coz 255hp at 8000 is probably 125 at 4000 and 60 at 2000 so it would only plane a jet ski
    ( there are 3 ltr sprint cars making 255hp at high revs)
     
  9. fpjeepy05
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    fpjeepy05 Senior Member

    Why does a boat have to plane at 2000 rpm? I understand a custom reduction gear would be necessary, but aside from that I don't see anything wrong with that. 6.0:1 would equal things out. I guess the limiting factor would be engine life. But the new Rotax 4tec's seem promising. Saw a guy posting on another forum with 582 hours on one. Its not commercial, but its a decent amount for the average recreational guy. Also Seadoo boats with that motor come with a 3 year warranty... expendable to 5...
     
  10. powerabout
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    powerabout Senior Member

    its not the reduction its the torque at that rpm
    If you make a huge reduction to get something on the plane then where is the rest of your rev range to go any faster.
    Ever wondered why jet ski and skiido's have custom engines.
    Brp and Yam make both and outboards, any crossover products, there's your answer???
     
  11. fpjeepy05
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    fpjeepy05 Senior Member

    I'm sure the manufactures have their reasons....

    But if twin 255hp 1503 SC IC motors can get a 23ft 3500lb sea-doo jet boat up to almost 60mph. And a 0-30mph in 2.5 seconds I don't think they would have much problem doing anything similar in a 23ft 3500lb cruiser, tender or Center Console. Jet pumps don't defy physics, the power is there. I'd be real scared doing 60 in a 23ft center console. So maybe a little extra reduction would be nice to slow it down a little. I admit getting the perfect prop might be difficult especially if all you care about it top end, but if cruise is important it would make it a little easier.
     
  12. powerabout
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    powerabout Senior Member

    Jets have lots of slip at low speed so the engines can get up to rpm and hence usually accelerate faster then same engine with a prop but with a prop you still have a gear ratio and pitch to alter that
    If you ran 1:1 with a normal prop forget it.
     

  13. fpjeepy05
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    fpjeepy05 Senior Member

    I think it would need at least a 4:1 gear reduction to be used effectively. But with such a gear, I think it would have more low end torque than a Mercruiser At that point is come down to proportionality. The Mercruiser is turning half the rpms with twice the torque. But if the Rotax has twice the gear reduction the prop sees the same thing. So considering that the engine with the flatter torque curve wins... which would generally be the supercharged engine. Not to mention it is a third the weight.

    So they don't sell them, because no one makes a gear?

    Or they don't want anyone else putting them in their boats, because then it takes away Seadoo's advantage?

    High rpms high wear? Yes, but the average boat owner uses their boat the same amount as their jetski.
     
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